CITROBACTER SPP.
Citrobacter species are straight, facultative anaerobic, Gram-negative bacilli and are typically motile by means of peritrichous flagellae and coliform bacteria in the Enterobacteriaceae family. This genus was proposed in 1932 by Werkman and Gillen. Before 1993, only three species, Citrobacter freundii, Citrobacter koseri (Citrobacter diversus), and Citrobacter amalonaticus, were recognized. C. freundii is the type species in this genus, and the later two species have been called other names. C. koseri was accepted to replace the name C. diversus by the Judicial Commission of the International Committee on Systematic Bacteriology in 1993. In the same year, Brenner et al. classified Citrobacter into 11 genomospecies by DNA hybridization: C.freundii, C.koseri, C. amalonaticus, C. farmeri, C. youngae, C. braakii, C. werkmanii, C. sedlakii, and unnamed genomospecies 9, 10 and 11. Later, genomospecies 9, 10 and 11 were named as C. rodentium, C. gillenii, and C. murliniae.
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| Citrobacter freundii |
Citrobacter species are commonly found in water, soil, food, and the intestinal tracts of animals and humans. Many Citrobacter infections are nosocomially acquired; however, they can also be community acquired. A large surveillance study demonstrated that 0.8% of Gram-negative infection was caused by Citrobacter spp. In the hospital settings, Citrobacter spp. might account for 3 – 6% of all Enterobacteriaceae causing nosocomial infection. In patient with Citrobacter infections, the bacteria can be transmitted vertically from mother or horizontally from carriers or other hospital sources. The infection may occur as sporadic cases or nosocomial outbreaks. Vertical or nosocomial transmission may account for the origin of bacteria in some sporadic cases, and transmission from carriers such as family members or other contacts accounts for others.
LABORATORY DIAGNOSIS
Patients with Citrobacter infection can be identified and confirmed only by culture. Citrobacter species can grow in various culture medium. All species identified as Citrobacter ferment glucose with production of gas. With few exceptions the organisms are motile and utilize citrate. Different species can be differentiated by biochemical tests. The interpretation of antimicrobial susceptibility testing follows the criteria used for Enterobacteriaceae.
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| GRAM STAINING |
The organisms probably colonize the oral cavity, lower gastrointestinal tract, or respiratory tract first. Later it may result into infection of various sites, including bacteremia and central nervous system (CNS) infection. Nosocomial outbreaks are largely due to gastrointestinal and hand carriage by hospital personneln.
The particular propensity of C. koseri for CNS is not well understood. A specific outer-membrane protein of 32-kilodalton was proposed to be related to the tropism of this organism for nervous tissues and causing meningitis and abscesses.
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| Citrobacter freundii on MacConkey |
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